A polycarbonate-polyorganosiloxane polymer (hereinafter sometimes referred to as “PDMS-PC”) has been attracting attention because of its high impact resistance, high chemical resistance, and high flame retardancy, and the polymer has been expected to find utilization in a wide variety of fields such as the field of electrical and electronic equipment, and the field of an automobile. In particular, the copolymer is being more and more widely utilized in the casings of a mobile phone, a mobile personal computer, a digital camera, a video camera, an electric power tool, and the like, and in other articles for daily use.
Meanwhile, in recent years, additionally high transparency has be required in terms of design and functions. For example, a mobile phone button preferably has high transparency so that a character or number printed on a back surface can be viewed more clearly. With regard to the casing of a mobile phone, digital camera, or mobile personal computer, an external appearance having a higher transparency is preferably obtained when the casing is colored with a dye or painted from its back surface. In addition, a material to be used in the window of a meter or the like required to have visibility or in a member required to have light permeability is required to have high transparency, not only in terms of design. Accordingly, a large number of improvements have been attempted with a view to imparting additional transparency to a material using the PDMS-PC.
It has been found that a PDMS-PC having a relatively short chain length, specifically, a PDMS-PC in which the repetition number (n) of organosiloxane units is 60 or less is desirably used for obtaining a PDMS-PC excellent in transparency. For example, there has been known a polycarbonate-polyorganosiloxane block copolymer in which the repetition number (n) of organosiloxane units is 60 or less and which is suitable for molding a transparent visor for a helmet (see Patent Document 1). In addition, a polycarbonate-polyorganosiloxane block copolymer in which the repetition number (n) of organosiloxane units is 30 has been known as a constituent component of a polycarbonate resin composition excellent in heat stability, flowability, and flame retardancy (see Patent Document 2).
However, transparency obtained merely by using a PDMS-PC having a short chain length is insufficient in some cases, and hence an improvement concerning a producing method for a PDMS-PC has also been investigated. A structure in which organosiloxanes are linked to each other through phosgene causes a reduction in transparency. Accordingly, the following method of improving the transparency has been known. A polycarbonate oligomer and a polydimethylsiloxane (hereinafter sometimes referred to as “PDMS”) are caused to react with each other in a state where phosgene is substantially absent to eliminate a structure in which the PDMS's are linked to each other through phosgene. For example, there has been known a silicone-polycarbonate block copolymer having a haze of about less than 10, the copolymer being obtained by producing a polycarbonate oligomer having a terminal chloroformate group with a phase-transfer catalyst and then adding a hydroxyaryl-terminated polydiorganosiloxane (see Patent Document 3). However, the copolymer has been inferior in both total light transmittance and haze to a polycarbonate obtained from bisphenol A (BPA) and phosgene (hereinafter referred to as “BPA-PC”), and its transparency has been insufficient as compared with that of the BPA-PC.
In addition, the following method of producing a silicone-containing copolycarbonate in which transparency is maintained in spite of a relatively long siloxane chain length has been provided. A hydroxy-terminated polycarbonate oligomer is brought into contact with a siloxane bischloroformate having a residual hydroxy terminal group at less than 10% under an interface reaction condition to produce a silicone-containing polycarbonate intermediate, and then the intermediate is caused to react with BPA, p-t-butylphenol (PTBP), and phosgene under an interface reaction condition to provide the silicone-containing copolycarbonate (see Patent Document 4). However, it cannot be said that the copolycarbonate has transparency comparable to that of the BPA-PC.
In addition, it has been known that excellent transparency and excellent physical properties are obtained by producing an aromatic bischloroformate while maintaining a pH within the range of about 3 to about 8 and using the aromatic bischloroformate in the production of a siloxane-polycarbonate copolymer (see Patent Document 5). More specifically, the ratio of the mol % of a chloroformate group to the mol % of a phenol-based terminal group of a polydiorganosiloxane is set to 4 or more. However, the copolymer has been inferior in both total light transmittance and haze to the BPA-PC, and its transparency has been insufficient as compared with that of the BPA-PC.
A method involving improving the reaction result of the PDMS has also been proposed as a method of improving the transparency.
For example, a method involving controlling the flow of a raw material to continuously produce a transparent polycarbonate-polyorganosiloxane block copolymer has been known (see Patent Document 6). Specifically, the method involves: continuously mixing a polycarbonate oligomer and a polyorganosiloxane under such a condition that an alkaline compound is substantially absent; then subjecting the mixture to a reaction in the presence of the alkaline compound to produce a polycarbonate-polyorganosiloxane copolymer oligomer; and then causing the resultant polycarbonate-polyorganosiloxane copolymer oligomer and BPA to react with each other in the presence of the alkaline compound. However, satisfactory transparency has not been obtained even by the method.
In addition, a polycarbonate-polyorganosiloxane copolymer characterized in that an average domain size is 5 to 40 nm, a normalized dispersity is 40% or less, and a total light transmittance is 88% or more has been recently reported (see Patent Document 7). However, such values can be achieved even by a conventionally known method (see, for example, Patent Document 8), and hence a further improvement in transparency, in particular, a reduction in haze has been desired.